Third ventricle

Floor

The floor of the third ventricle extends from the optic chiasm anteriorly to the cerebral aqueduct posteriorly. The anterior portion of the floor is formed by the hypothalamus, while the posterior portion is formed by the midbrain (mesencephalon). Going from the anterior to posterior, the structures that comprise the floor of the third ventricle are the:

• Optic chiasm
• Infundibulum of hypothalamus
• Tuber cinereum
• Mammillary bodies
• Posterior perforated substance
• The anterior part of the tegmentum of the midbrain

When the floor of the third ventricle is viewed from above, there is a noticeable prominence in the anteriormost part of the ventricle, formed by the optic chiasm. Note that the optic chiasm is located at the junction of the floor and anterior wall of the ventricle. Posterior to the optic chiasm, there is a prominence formed by the mamillary bodies. Between these two prominences, the cavity of the third ventricle extends into the pituitary stalk, forming the infundibular recess. The posterior half of the floor (posterior to the mammillary bodies) lies above the posterior perforated substance and tegmentum and it is generally smooth and unremarkable.

Test your knowledge on the ventricular system of the brain with the following interactive quiz.

Roof

The roof of the third ventricle extends from the foramen of Monro anteriorly to the suprapineal recess posteriorly. The roof lies immediately below the body of fornix.

The roof has for separate layers which include:

• Neural layer
• Tela choroidea (two layers)
• Vascular layer

The most superficial layer is the neural layer, formed by the body and the crura of the fornix. Below this layer, there are two thin membranous layers of tela choroidea. The tela choroidea is semi-transparent double-layered structure derived from the pia mater and we’ll speak about its function further in the text.

In between the layers of the tela choroidea, there is a vascular layer that mainly consists of medial posterior choroidal arteries and their branches.

Walls

The third ventricle has four walls in total; anterior, posterior, and two lateral walls.

Cavity and recesses

The cavity of the third ventricle extends into four recesses:

• The supra-optic recess is located superior to the optic chiasm and inferior to the lamina terminalis.
• The infundibular recess is found on the floor of the third ventricle, between the optic chiasm and the mamillary bodies. It extends inferiorly, into the pituitary stalk.
• The pineal recess extends posteriorly. More specifically, it projects into the pineal body, between the cranial and caudal lamina of the pineal gland.
• The suprapineal recess is located between the superior portion of the pineal gland and the inferior aspect of the tela choroidea in the roof of the ventricle. It extends posteriorly, behind the posterior wall of the third ventricle.

Choroid plexus and tela choroidea

Tela choroidea is the vascular pia matter which is closely connected to the ependymal lining of the ventricles. In the third ventricle, the tela choroidea is, in fact, embedded in the multi-layered roof of the ventricle. The tela choroidea gives rise to the choroid plexus.

The choroid plexus is a collection of capillaries that produce and secrete the CSF into the ventricular system of the brain. The epithelium of the choroid plexus constitutes the blood–CSF barrier.

The blood for the choroid plexus is mainly supplied by the anterior choroidal branch of the internal carotid artery and choroidal branches of the posterior cerebral artery. The venous blood is drained by a single choroidal vein. When viewed from the sagittal section, the choroid plexuses of the lateral and third ventricles are continuous.

Function

The function of the third ventricle is to produce and secrete the CSF, as well as to contribute to the continuous circulation of the CSF through the ventricular system. This way, the ventricle participates in cushioning and protecting the brain from injury, as well as in the transport of nutrients and waste in and out of the neural tissue. If the CSF flow through the ventricles is obstructed for any reason (e.g. a tumor that blocks the foramina through which the ventricles communicate), the CSF starts to accumulate and results in hydrocephalus.

Do you want to know more about the ventricles of the brain? Check out our study unit: